The move sets up what likely will be a fiercely contested special election, the second in Florida this year after the death of C.W. Bill Young in Pinellas County.

Radel, 37, announced his resignation in a letter to House Speaker John Boehner. "It is my belief that professionally I cannot fully and effectively serve as a United States Representative to the place I love and call home, Southwest Florida."

Radel also wrote to Gov. Rick Scott and Florida Secretary of State Ken Detzner. Scott will have to set an election date.

Rumors began to pick up last week that Radel, who bought 3.5 grams of cocaine from an undercover police officer in late October, would soon step down or announce he would not seek re-election.

Radel had refused calls to resign and returned to work recently after about a month in rehab, vowing to regain the trust of voters and his colleagues.

But Radel also faced extreme odds of hanging on given a primary and an Ethics Committee investigation that could have delved deeper into his actions in Washington, where he arrived in January 2013, and the personal cost that was mounting with lawyers.

By quitting, Radel likely ended the inquiry.

The group Citizens for Responsibility and Ethics in Washington insisted it continue. "We know Rep. Radel shared his cocaine with others. Who, exactly?" the group said.

Former state Rep. Paige Kreegel has already announced his candidacy, and others, including state Sen. Lizbeth Benacquisto, are expected to get in the race. The district is solidly Republican, so the primary winner will likely decide the outcome.

The seat was previously held by Connie Mack IV, who vacated it to run for U.S. Senate in 2012. Mack is a possible contender as well. Even Fox News host Sean Hannity's name has surfaced.

A divisive primary was already shaping up. A super PAC supporting Kreegel recently ran a TV ad against Benacquisto, accusing her of campaigning for the seat while still seeking re-election to the Legislature. Benacquisto aired her own TV ads that do not specifically name an office but paint a warm portrait.

A statement she released Monday indicated she is looking at the race.

"Trey, his wife, Amy, and their precious son have been and will remain in my prayers," Benacquisto said. "This announcement also makes it clear that southwest Florida families will soon choose a new voice to represent them in Congress. I will consider the best way I can be of service to Florida and our region."

Mack was less clear on his intentions. "Now it's time for southwest Florida to elect a new congressman who will be a tireless champion of our shared mainstream conservative values," he said, also expressing sympathy for Radel, who won a crowded Republican primary last year.

Kreegel said, "It's time to move forward. I will focus on common-sense solutions that are based on conservative principles and conservative values."

Radel got caught up in a broader cocaine trafficking investigation. A dealer said a regular customer was a congressman and police set up an undercover operation.

According to court documents, on Oct. 29 Radel and an acquaintance went to a restaurant in the Dupont Circle area of Washington and met a man, who was an undercover police officer. Radel said he had cocaine at his apartment and invited the men, who declined. But the officer offered to sell Radel 3.5 grams of cocaine for $250.

As federal agents approached, Radel dropped the cocaine in the street.

"The defendant agreed to speak with federal agents about what had just taken place and invited them to his apartment," court documents state. "There, he voluntarily admitted that he had purchased the cocaine. The defendant also retrieved and provided to the agents a vial of cocaine that he had in his apartment."

The news did not surface for almost three weeks, when the case showed up in court. (Radel's given first name is Henry, so it would not have stood out.)

Radel continued to vote and tweet as if nothing happened. He held a re-election fundraiser at a country club in Naples.

On Nov. 20, he pleaded guilty in a Washington court to misdemeanor cocaine possession. Had he been arrested in Florida, he would have faced a felony.